Synergetic Effects of PdO Species on CO Oxidation over PdO–CeO₂ Catalysts

The PdO/Ce 1– x Pd x O 2−δ catalyst prepared by a solution-combustion method contained free surface PdO species and PdO species in Ce 1– x Pd x O 2−δ solid solution, whereas the PdO/CeO 2 catalyst prepared by an impregnation method contained only free surface PdO species. The free surface PdO species could be removed by nitric acid. Contributions of the PdO species to catalytic CO oxidation were quantitatively evaluated. The free surface PdO species in the PdO/Ce 1– x Pd x O 2−δ catalyst had the highest activity (969.3 μmol CO g Pd –1 s –1 ), those in the PdO/CeO 2 catalyst had medium activity (109.0 μmol CO g Pd –1 s –1 ), and the PdO species in the Ce 1– x Pd x O 2−δ solid solution had the lowest activity (13.2 μmol CO g Pd –1 s –1 ). Synergetic effects of PdO species were responsible for the enhanced reactivity of the PdO/Ce 1– x Pd x O 2−δ catalyst, as the free surface PdO species provided CO chemisorption sites and the Ce 1– x Pd x O 2−δ solid solution generated more oxygen vacancies for oxygen activation.